1. Field of the Invention
The present invention generally relates to an image forming apparatus and image forming method, and specifically relates to a supporting mechanism that regulates positions of a plurality of latent image carriers in an image forming apparatus.
2. Description of the Related Art
In the image forming apparatus of the type explained above, if there is an error in arrangement pitches of the latent image carriers, the visible images obtained through development on the surface of the respective latent image carriers are transferred onto the transfer member of which the images are displaced in position with respect to each other. This causes a displacement in superposition of the respective visible images on the transfer member.
Also, in general, in the image forming apparatus of this type, a latent-image writing device that writes a latent image on the latent image carriers is placed in a direction orthogonal to a direction in which the latent image carriers are arranged (hereinafter, “orthogonal direction”) so that the latent-image forming apparatus faces the latent image carriers. In this configuration as a device that writes a latent image through exposure, as each latent image carrier has a different distance from the latent-image writing device, an error can occur in exposure writing magnification between the latent image carriers. This error causes different dot sizes on the latent image carriers to disturb the final visible image on recording sheets.
Japanese Patent Application Laid-open No. 2002-189324 discloses an image forming apparatus having a plurality of openings formed on its side panel and aligned with predetermined pitches, the openings each respectively corresponding to a latent image carrier fitting therein. In this configuration, the latent image carriers fit in the openings provided on the side panel with the predetermined pitches so as to position accurately both of an arrangement direction and an orthogonal direction. This can decrease an error in arrangement pitches of the latent image carriers and a displacement in superposition of the respective visible images due to such an error. This also can decrease an error in distance between each latent image carrier and the latent-image writing device and an image disturbance due to such an error.
In the image forming apparatus, each latent image carrier fits in the relevant opening, which is a positioning portion on the side panel, intending to position accurately in both of the arrangement direction and the orthogonal direction. Therefore, it cannot individually position in both of these directions. This hinders to reduce cost and weight of the image forming apparatus. In general, it requires the latent image carrier a higher positioning accuracy in the arrangement direction than in the orthogonal direction. This is why a displacement in superposition due to an error in arrangement pitches causes more severe image quality decrease than an image disturbance due to an erroneous distance between latent image carriers and the latent-image writing device. For this reason, it is required to use a positioning portion that is formed of a high-stiffness material with less distortion and deformation such as metal in the arrangement direction to regulate the movement of each latent image carrier for positioning with a relatively high accuracy. On the other hand, it is not necessarily required to use a positioning portion that is formed of a high-stiffness material in the orthogonal direction to regulate the movement of each latent image carrier, so that even a positioning portion formed of a low-stiffness material such as resin can satisfy a desired positioning accuracy. In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-189324, the latent image carriers are positioned in both of the arrangement direction and the orthogonal direction by single positioning portion of the openings. Therefore, it is impossible to individually select a material of the positioning portion for each of the directions according to the required positioning accuracy. As a result, the whole side panel is formed of sheet metal, which is a high-stiffness material, to meet a higher positioning-accuracy required in the arrangement direction. This leads the apparatus an increase of cost and weight.